1. Field of the Invention
This invention relates to pressure relief devices, assemblies, and components, as well as methods of forming the same.
2. Background of the Invention
Many types of pressure relief devices exist in the art. These pressure relief devices may include, for example, explosion panels, rupture disks, vacuum supports, and valves. An explosion panel is one type of pressure relief device that is typically used to provide emergency pressure relief under deflagration conditions in an environment such as, for example, a silo or a dust collector. An explosion panel may be subject to both a positive pressure differential and a negative pressure differential. In a positive pressure differential, the pressure within the environment is greater than the external pressure. In a negative pressure differential, the external pressure is greater than the pressure within the environment. In most circumstances, it is desirable for the explosion panel to open when exposed to a predetermined positive pressure differential and to withstand a negative pressure differential.
Various efforts have been made to improve the vacuum resistance of explosion panels. For example, the explosion panel may be shaped to provide a greater resistance to a negative pressure differential than a positive pressure differential. This may be accomplished by forming the explosion panel with a domed shape and exposing the concave surface to the pressure within the environment. This configuration provides greater structural integrity under negative pressure differentials than under positive pressure differentials. Thus, the explosion panel may be configured to open when subject to a predetermined positive pressure differential yet be able to withstand a greater negative pressure differential.
In another method of improving vacuum resistance, a separate “vacuum support” may be included with the explosion panel assembly. This vacuum support may be attached to the concave side of the explosion panel to improve the vacuum resistance. However, an explosion panel should open quickly and completely in response to the predetermined positive pressure differential. In many cases, the additional weight of a vacuum support will inhibit the ability of the explosion panel to quickly and completely open. In addition, the inclusion of a vacuum support may increase the costs associated with manufacturing the explosion panel.
To minimize explosion panel mass, designs that do not require a vacuum support are desirable. Higher mass vents will be less responsive to a dynamic pressure rise. International Standards may limit the mass permitted; NFPA 68 has a mass limit of 2 ½ pounds per square foot. Alternatively, standards may require that ‘vent efficiency’ be experimentally determined resulting in a greater vent area being required for designs that are lower in efficiency. Higher mass typically results in lower vent efficiency.
Various methods may be used to control the predetermined positive pressure differential at which the explosion panel will open. For example, a series of slits may be cut into the explosion panel to define a series of “tabs.” The slits may be cut into the domed section of the explosion panel or the flange section of the explosion panel. These tabs are configured to fail in tension when the explosion panel experiences the predetermined positive pressure differential. The number and size of the tabs will control the pressure differential at which the explosion panel will open. Accordingly, the slits must be carefully cut to ensure that the resulting tab has the appropriate size.
These slits may, however, reduce the vacuum resistance of the explosion panel. When the slits are cut into the explosion panel, the structural integrity of the explosion panel is weakened. Thus, the explosion panel may fail in the area of the slits when exposed to a negative pressure differential. Even if the explosion panel is exposed to a negative pressure differential that does not cause the panel to fail, repeated pressure cycles may fatigue the tabs and thereby alter the pressure differential at which the explosion panel will open.
The pressure differential at which the explosion panel will open may also be controlled by securing the explosion panel to the environment with plastic bolts. The plastic bolts are configured to break when the explosion panel is subject to the predetermined pressure differential. However, the operating conditions of the plastic bolts have a direct impact on the material strength of the bolt. Varying climate conditions may alter the material strength of the plastic bolts and, thus, the pressure differential at which the explosion panel will open. A plastic bolt may also fail at a much higher load under dynamic deflagration venting conditions making prediction of behavior unreliable.
When a pressure relief device, such as, for example, a rupture disk, an explosion panel, or a vacuum support, is exposed to the predetermined pressure differential, a portion of the pressure relief device will typically tear to create an opening. Safety considerations dictate that the opening material should remain attached to the nest of the pressure relief device, instead of fragmenting. To prevent fragmentation, the pressure relief devices typically include an unweakened hinge area. When the pressure relief device opens, the unweakened hinge area prevents fragmentation of the pressure relief device. However, when the pressure relief device experiences a pressure differential that is significantly greater than the predetermined opening pressure differential or a sustained turbulent flow, the hinge area has a tendency to tear, thereby allowing the pressure relief device to fragment.
There is a need in the industry for a pressure relief device that is capable of withstanding vacuum pressure, has a low mass and therefore improved dynamic performance, that will release at the predetermined pressure regardless of the operating environment in which it is placed, that is resistant to operating pressure cycles, and opens without fragmentation. Different aspects of the present invention provide a solution to each of these identified problems.